The invention relates to a sealing structure for a rock drill bit and, more particularly, to a sealing structure that includes a diaphragm as a fluid pressure compensator and a relief valve between lubricant and mud regions.
Rock drill bits have multiple rotating heads which are caused to rotate by rotation of the drill pipe stem. The rotating heads are mounted on bearings which receive lubrication from a reservoir, with rotary seals preventing escape of the lubrication. Due to fluctuations in the pressure differential between the lubricant and the mud surrounding the rotating heads, a pressure relief mechanism has to be provided to lower lubricant pressure when the differential exceeds an amount that may damage the rotary seals; such damage results in both downtime and high repair cost.
The pressure relief mechanism for such lubrication systems normally includes an elastomeric compensating diaphragm as well as an associated means that allows lubricant to flow through or around the diaphragm if the pressure differential thereacross exceeds a predetermined value. A variety of such diaphragms are shown, for instance, in the following U.S. Pat. No. 3,847,234 (Schumacher); U.S. Pat. No. 4,161,223 (Oelke); U.S. Pat. No. 4,727,942 (Galle); and, U.S. Pat. No. 5,072,795 (Delgado). In Schumacher, the diaphragm is generally planar with corrugations, and has a central orifice through which lubricant can pass if the pressure differential exceeds a predetermined value. In Oelke, lubricant escapes around the edges of an accordion-shaped diaphragm if the pressure differential exceeds the force exerted by a belleville spring holding a cover cap against the base of the diaphragm. In Galle, the diaphragm is centrally sealed in a chamber and movable between opposite ends of the chamber, depending on the relative pressure differential across it. A central portion of the diaphragm has a perforation that is normally closed but is forced open if the lubricant pressure exceeds the mud pressure by more than a maximum differential; the perforation does not open to allow mud through, no matter how high the pressure differential. In Delgado, a self-sealing puncture is included in the diaphragm and opens when the pressure differential exceeds a critical value to allow either the mud or the lubricant having the higher pressure to pass through the puncture; once the pressure differential reduces below the critical value, the puncture self-seals to prevent further fluid passing through it.
It would be advantageous to be able to adjust a maximum pressure differential across a diaphragm without requiring a replacement of the diaphragm.
It would also be advantageous to be able to adjust such maximum pressure differential in a simple and rapid manner.
In one aspect, the invention is a sealing structure for a rock drill bit. The sealing structure includes an elastomeric diaphragm for installation in the drill bit so as to separate lubricant from drilling mud. The diaphragm has an aperture therethrough, and a plug is disposed in the aperture to form a mechanical seal with a wall thereof. The plug has a bore therethrough which is closed by the expanded diaphragm. Excess pressure in the lubricant is communicated via the bore to the aperture wall to deform the wall locally and vent the excess pressure.
The plug preferably forms an interference fit with the aperture of the diaphragm.
The bore of the plug preferably has an axial segment and a radial segment. The axial segment extends from an open plug end on the lubricant side of the diaphragm to a closed plug end on the mud side of the diaphragm. The radial segment comprises one or more channels each of which connects the axial segment of the bore to a corresponding side opening on the plug.
In a preferred form, the diaphragm when unstressed is generally cup-shaped, an inner side of the diaphragm being the lubricant side, and wherein the diaphragm is adapted to be secured by its larger open end to the rock drill bit. In a more preferred form, the diaphragm may have a generally frustoconical configuration, and includes a tapering sidewall integrally connected at its smaller end to a generally circular base.
The plug may preferably extend through the diaphragm at a central position on the diaphragm base.
The plug may engage with a collar that is mounted on an inside surface of the diaphragm base. Preferably, the plug engages with the collar by means of complementary threads. Preferably, the open plug end is configured for accepting a rotatable end of a drive tool for rotating the plug relative to the collar.
A series of protrusions may surround an entrance to the aperture on an outside surface of the diaphragm base. The larger open end of the diaphragm has a toroidal shape which is adapted to be held between mating parts of the rock drill bit for holding the diaphragm in position on the bit.
In any of the foregoing forms of the sealing structure, the diaphragm may be elastomeric, the plug may be metallic, and the collar may be metallic. The lubricant used in the sealing structure may be grease.
The aperture wall may deform to allow lubricant to pass through the sealing structure at an excess pressure in the range between 3.5 and 35 kg/cm2 (50 and 500 psi),
In another aspect, the invention is one of the foregoing forms of the sealing structure, in combination with the rock drill bit.
Preferred features of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: